These look quite nice, have you tried a Fiio E17? I would be interested in whether you think there are any advantages yo one over the other? The Fiio is cheaper and about the same size, also no potentiometer though.

The FiiO E17 is now on the way as is an Audioengine D1 - both of these are USB DAC's with headphone amps. In the case of the Audioengine D1 the power comes entirely from the USB, exactly like the HRT HeadStreamer. In those latter two cases a low-efficiency headphone will probably not be able to play wide dynamic tracks at full volume without distorting on some of the stronger peaks. I've heard a couple of examples with the HeadStreamer and the fairly efficient ATH M50, so I'd advise caution with headphone amps powered entirely by the USB using low-efficiency headphones.

Here is a comparison I did today between the HRT iStreamer and HeadStreamer: